Wireless communications devices are used for a wide variety of applications, e.g., hearing aid, medical devices. In some of the wireless communications devices, a receiver uses a mixer for mixing a signal received from an antenna with a local oscillator (LO) signal. The mixer generates an IF (intermediate frequency) signal having a frequency in a lower frequency band. However, by the mixing operation, image band and desired band are translated into the same IF frequency. In order to remove the undesired frequency components, it is required to employ an image rejection mechanism in the receiver side.
FIG. 1 is a diagram illustrating a conventional 50 ohms and low-IF receiver. The receiver 10 of FIG. 1 includes a matching block 12 coupled to a 50 ohms antenna 14 for receiving communication signals, a low noise amplifier (LNA) 16, mixers 18 and 20, a 90° phase shifter 22 coupled to the output of the mixer 20, and an adder 24 for adding the output from the mixer 18 and the output from the 90° phase shifter 22. The matching block 12 converts the receiver input impedance to 50 ohms to match the receiver input to a 50 ohm conventional antenna. The receiver 10 includes a band pass filter (BPF) 26 coupled to the output of the adder 24, a limiting amplifier 28, and a demodulator 30 for recovering information. The output from the LNA 16 is mixed with quadrature phases of a local oscillator 40. Signals from two branches are added by the adder 24, then an undesired signal is cancelled.
The local oscillator block 40 includes a crystal oscillator 42, a synthesizer 44, and a 90° phase shifter 46. The local oscillator 40 produces quadrature local oscillator signals 50 and 52 that are 90° phase shifted with respect to each other. The local oscillator signals 50 and 52 are provided to the mixers 18 and 20, respectively.
FIG. 2 is a diagram illustrating another conventional 50 ohms and low-IF receiver 60. In receiver 60, a polyphase filter 62 is provided to the outputs of the mixers 18 and 20. One of the outputs from the polyphase filter 62 is directly supplied to the BPF 26. The polyphase filter 62 rejects the image signal and generates two wanted signal with 90 degree phase deference at the output. One of these signals can be connected to the next stage.
Some improvements have been done to the receivers/transceiver in order to reduce power consumption for portable devices. However currently available receivers/transceivers have still relatively high power consumption and large size.
There is a need to provide a low-power, compact image reject receiver, transmitter or transceiver having the receiver and the transmitter.